Undying hate - drop rate and is defeating Vessel of Kulemak at full power required?

Hey,

Dropped my first Kulemak's Invitation in SSF. Main drop I want from the fight is the Undying Hate. So, my questions are:

Do you need to defeat Vessel of Kulemak at full strength for a chance to drop the timeless jewel? If there is a chance to drop it at all difficulties, does the full strength make the drop more likely?

To which existing boss would you compare Vessel of Kulemak or the fight in general in difficulty?

As someone who's beaten him over 120 times I can say that trying to farm him for the undying hate jewels is not a good idea. I'm not sure if they changed something in the past couple days but when I first started fighting him I got 2 undying hate jewels out of like 10 fights. That sounds about right...now I've fought him almost 100 times since and have gotten none. I even dropped my stats and health just to slap on more rarity and at 345 rarity I'm still not getting a change in the drops. Not sure if rarity makes no difference on the boss and if they changed something but it sucks. And to answer your question, yes you have to be at the Uber boss to get the diamond.

Yeah, looks like jewel chances have changed